2007 Grand Prix, 3.8L, SC - No Cabin Heat/Contaminated Coolant/Overheating

[Gary1968]

If anyone out there can help me, I'd sure appreciate it. I have multiple symptoms, but can't tie them together (if they need to be).
Low Cabin Heat
Outside Air Temp. = 51°F
Discharge Air Temp. = 102°F
Coil Inlet Hose = 160°F
Coil Outlet Hose = 135°F
Coil backflushed with no improvement
Coolant
Coolant just flushed but looked and smelled bad prior to flush
Contaminants (oil?) still present in new fluid (darker but not as bad as before flush)
System vented of air
Oil
Oil clean but just changed less than a week ago
Car has used oil for over two years not, but no external leaks nor smoking
Periodically Overheats
Thermostat replaced 6 weeks ago
Other
Transmission Fluid clean
Power Steering Fluid clean
No external leaks
No smoke at all
No loss of power
Car runs smoothly

[Fivefingerdeathpunch]

Is that with the car fully warmed up??

One heater core hose should be at or very near the coolant temp of the motor when warm, so roughly 190-195º. You might still have sludge build up in the core.

If you replaced the dexcool coolant, good luck getting all the stuff out on just one flush. It might take 3-5 flushes to get all that junk of the system. Dexcool and air do not like each other, thus the oil film or sludge you can see.

Burning oil is just something some 3800's do, the bonneville we have and the grand prix (both 2002's) burn oil. I've changed the PCV on each and they still do the same thing, even replaced the intake manifold on one for prior issues but it didn't help anything.

1 quart per 3,000 miles is normal per GM, you won't notice smoke with that. And all cars do burn oil, impossible not too but just some more than others.

[Mirraco]

It does take a long time to get the nasty dexcool out.

I got a impala. It has the 3400 engine. The owner told me after I bought it what oil he was using. It was like 40w15 diesel oil. I was like what the hell? I normally run 5w30 in my cars. I threw 10w30 in it to compensate for the oil he was using.

I drove 100 miles, and it was black. Changed again. Drove 250 miles, put seafoam into crankcase and it was black as black again. I have about 50 miles on this last change, been working on the suspension.

Also, i should say most gms will use oil after awhile. My 08 g6 with 103K miles uses about 1/2 to 1 qt of oil every 3000-4000 miles. It matters how long it was since I changed it. I compensate with putting 5qts instead of the recremended 4.5qts.

[rottonj]

Could be air in the cooling system still or a partially plugged heater core. Be sure fans are operating if overheating.